1. Field of the Invention
The present invention relates to a plastic impression tray for a local area to be used for taking an impression of a local area within an oral cavity as a preparation stage of producing a partial denture, a bridge, an inlay or the like as a prosthetic appliance in dentistry, wherein the impression tray for a local area can prevent a set impression material from floating upward from the tray and being peeled off from the tray, and therefore can take an impression with high accuracy.
2. Description of the Conventional Art
In a dental care, an impression material such as a silicone impression material or an alginate impression material is used for taking an impression within an oral cavity as a preparation stage of producing a prosthetic appliance. For inserting and retaining such an impression material within the oral cavity, an impression tray is used. In the impression taking, the impression material is loaded in the impression tray so as to be inserted into the oral cavity of a patient, the impression material is pressed into the oral cavity of the patient to be carried out the impression taking, the impression material is set, and the impression material is thereafter taken out of the oral cavity of the patient integrally in a state in which the set impression material is retained in the impression tray.
Large force is necessary at a time of taking out the set impression material from the oral cavity. As a result, there is a problem that the set impression material floats upward from the impression tray and is peeled off from the impression tray, so that an air gap is generated between the impression tray and the impression material. In the case that the air gap is generated, it is often the case that the impression material deforms from the initial set state, and a precision of the prepared dental prosthetic appliance is significantly lowered. Further, since the impression material has a property that the impression material is shrinked by a reaction and a volatilization of a water content after being set, the impression material is greatly affected by the shrinkage in the case that the impression material is peeled off from the impression tray, whereby a precision of the dental prosthetic appliance is further lowered. Accordingly, for the impression tray, there becomes important a performance that the impression material inserted into the oral cavity of the patient so as to be set can be taken out from the oral cavity of the patient without being deformed.
Generally, the impression tray is provided with a retention hole or a groove for mechanically retaining the set impression material. In other words, the impression material paste before being set enters into the retention hole and is set, whereby the set impression material and the tray are mechanically engaged. Particularly, since the set impression material tends to be peeled off in a peripheral portion of the tray, an application of forming an undercut portion (a rib) along the peripheral portion of the tray is carried out. However, since a sufficient amount of impression material cannot enter into the undercut portion, the peeling tends to be generated in the peripheral portion. Further, the impression tray is made of a metal or a plastic, and the metal tray can be easily formed an undercut portion having a height of 1.5 to 2.5 mm in the peripheral portion by processing as mentioned above. On the other hand, since the plastic tray is affected by a metal mold which is used for producing the tray and the molded tray cannot be taken out, the plastic tray cannot be formed an undercut portion having an effective height. Therefore, particularly in the impression tray made of a plastic, a problem that the peeling from the tray peripheral portion of the tray is large, and the deformation becomes larger occurs.
As the impression tray made of a plastic, there is a dental impression tray which is provided with a bottom wall, and inner and outer walls for forming an impression material retaining concave portion which is similar to a tooth row shape and is formed as a curved shape in a plan view, is provided with a handle portion which extends forward from side ends of front teeth in the bottom wall or the outer wall, and is provided at least in the inner and outer walls of the concave portion with a lot of impression material retention holes which inhibit a relative movement to the impression material over a whole surface (refer, for example, to Japanese Unexamined Patent Publication No. 2000-135227; hereinafter referred to as Patent Document 1). Patent Document 1 exemplifies a structure in which impression material retention holes provided in inner and outer opposed walls are elongated holes which are elongated shaped in a vertical direction and penetrate in an inward and outward direction of the concave portion (refer to claim 2), and the impression material retention hole provided in a front teeth side of the outer wall is provided in the bottom wall of the concave portion or the handle portion, or both of them and is communicated in a penetrating manner (refer to claim 3). However, Patent Document 1 shows only the structure in which a distance between the adjacent elongated holes in the drawing is very short in the impression material retention holes which are provided in the inner and outer opposed walls and elongated in the vertical direction as mentioned above, and any disclosure of a distance between the adjacent elongated holes and a width of the elongated hole does not exist in Patent Document 1. Further, an impression material retention hole having a circular cross sectional shape is provided in the bottom wall, however, a hole diameter and a distance between the adjacent impression material retention holes having the circular cross section shape are not disclosed at all. Further, the disclosed tray is provided only for taking a whole impression of the upper jaw and the lower jaw within the oral cavity, and there is not disclosed a structure for taking the impression of the local area within the oral cavity as a preparation stage of producing the partial denture, the bridge, the inlay or the like.
Therefore, it is unknown from Patent Document 1 mentioned above what dimension of the impression material retention holes can sufficiently retain the impression material. The impression material retention holes include the impression material retention holes which are provided in the inner and outer opposed walls and are elongated shaped in a vertical direction and the impression material retention holes which are provided in the bottom wall and have the circular cross section shape, in the tray for taking an impression of the local area within the oral cavity as the preparation stage of producing the partial denture, the bridge, the inlay or the like. Further, any means for removing a defect that the impression material in both ends of the tray tends to be peeled off does not exist.